A student has an unknown solution that may contain one or more of the following ions: carbonate ions, sulfate ions, or halide ions. The student adds dilute hydrochloric acid to a sample of the solution and observes effervescence. A second sample is then tested with acidified silver nitrate solution, producing a cream-coloured precipitate. Explain what the student can conclude from these two test results.

Eduqas GCSE Chemistry — C8 Chemical analysis · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

The effervescence when dilute HCl is added shows that carbonate ions are present, releasing CO₂ gas.
The second test with acidified AgNO₃ gives a cream‑coloured precipitate, indicating the presence of bromide ions.
The acid must be added first to remove carbonate ions; otherwise CO₂ could interfere with the halide test and give a false result.

Examiner tips

  • State the effervescence indicates CO₂ from carbonate.\nMention the cream‑coloured AgBr precipitate.\nExplain the need to remove carbonate first to avoid interference.\nUse correct terminology: carbonate, bromide, CO₂, AgBr.

Common mistakes

  • Claiming the precipitate is silver chloride instead of silver bromide.\nForgetting to mention that CO₂ is the gas released.\nNot explaining why the acid must be added first.

Mark scheme (4 marks)

  1. The effervescence with dilute hydrochloric acid shows that carbonate ions are present
  2. The gas produced is carbon dioxide
  3. The cream-coloured precipitate with acidified silver nitrate solution indicates bromide ions are present
  4. The acid must be added first to remove carbonate ions before testing for halide ions, to avoid a false positive

Key terms in this question

carbonate ions · halide ions · silver nitrate solution · precipitate · effervescence

Related

More Chemical analysis questions

▶ Try answering this question with AI marking (free) →